The present invention relates to a circuit for producing an alternating voltage in an electronic device which is connected to a d.c. voltage via the remote feed.
Such a circuit can be used for example to produce the ringing voltage in the "PCM 2" method used by the German Federal Post Office in which two spatially adjacent subscribers of the public telephone network are connected to the central office by one connection line. The further explanations will apply--as representative for another possible use of the invention--to this system. The letters "PCM" stand for the well-known "pulse code modulation method" by which communication signals are transmitted as digital signals.
In the PCM 2 method, the analog signals are digitalized in the central office and sent out on the subscriber's line. At the end of the subscriber's line there is installed a device by which the incoming digital signals are converted back into analog signals. This applies of course also to the opposite direction. The two subscribers are connected to the device by connecting lines.
Because of the digital transmission of the signals, the two subscribers can telephone at the same time without interferring with each other. The two streams of signals are in this connection "interleaved" by known technique at the start of the suscriber's line and separated again at the end thereof. Since the ringing voltage cannot be transmitted over the subscriber's line due to the digital transmission of the signal, it must be produced in the device.